The
Georgia Botanical Society

The
geology of Track Rock Gap has spawned two rarities in Georgia: ancient, undeciphered
petroglyphs, and the beautiful, threatened Fringed Gentian (Gentianopsis
crinita). The geology has also caused the formation of peculiar soils
that in turn support unusual plant communities. The interesting relationships
between the geology, soils, and plants found in places like Track Rock Gap
are described below.

Click for more information on the Fringed
Gentian from the Georgia Plant Conservation Alliance.

Geology
Here at Track Rock Gap in the Blue Ridge Mountains, you are actually standing
on rock that was once below an ocean's crust. Formed at great depths, it
was thrust upon the continent during closing of the pre-Atlantic ocean (Iapetus)
and the formation of Pangaea roughly 300 million years ago. The
minerals composing the Earth, from the core to the crust, are sorted by
density; that is, as the Earth formed, heavy minerals migrated toward the
core and lighter minerals frothed to the Earth's surface. Thus, the Track
Rock Gap rocks - formed at great depths - contain dark, heavy minerals such
as olivene, pyroxene, serpentine and talc.(Serpentine is formed when pyroxene
and olivine minerals, common in peridotite rock beneath the ocean crust,
are heaved up by plate tectonics and altered to serpentine by ocean water
and decreased heat and pressure. Rocks that abound with this mineral are
called serpentinites. Soapstone, the talc-rich material upon which the petroglyphs
are etched, is a soft, carvable rock related to serpentine.)

These
minerals have high amounts of the elements of magnesium and iron, and are
called ultra-mafic rocks.
(The word mafic stands for magnesium (ma), and iron, or ferrum (f).
Ultra-mafic means particularly high amounts of these elements.) Often,
other heavy elements, such as nickel and chromium, are present in ultra-mafic
rocks.

The
area surrounding Brasstown Bald is a melange of mafic and ultra-mafic minerals,
creating a mosaic of geologic and soil conditions. The mafic rocks may create
fertile, nutrient rich soils, while the ultra-mafic rocks may create conditions
hostile to many plants.

Soils

Ultra-mafic
rock outcrops are not common on the earth's surface, and the soils they
form - often called serpentine soils by ecologists - are unusual, with several
defining characteristics. First, they tend to be low in plant nutrients
such as phosphorus, potassium, nitrogen and calcium. Second, they have unusually
high magnesium levels and high magnesium to calcium ratios. The high levels
of magnesium in the soil block most plants' abilities to take up other nutrients,
especially calcium. Third, nickel or chromium can be present in levels toxic
to plants. Fourth, the soils contain minerals formed deep in the earth that
are no longer in equilibrium with their surroundings and are easily eroded.
The erosion creates a distinctive, pocked landscape described by Charles
Wharton as looking as though meteorites have hit it. These eroded, thin
soils do not retain very water well. Finally, these soils are typically
low in clay. One clay that does form, montmorillite, binds water to it so
tightly that vegetation cannot access the water.

All of
these factors render the serpentine soils low in both moisture and nutrients,
creating difficult conditions for plants. Hence, the vegetation is often
thinly dispersed, so nitrogen and organic humus cannot build in the soil
and the area remains open and hot, perpetuating the unfavorable conditions.

Plant
adaptations and communities

The
difficult environment - sometimes called the "serpentine syndrome"
- encourages plants that have evolved special adaptations to low moisture
and nutrient levels. Some plants have developed greater efficiencies in
absorbing calcium; others, such as Post oak (Quercus stellata) and
Blackjack oak (Quercus marilandica) selectively accumulate calcium.
Little Bluestem grass (Schizachyrium scoparium) rolls its leaves
to minimize water loss. Some species have a thick covering of hair on their
stems and leaves to reflect excessive sunlight and trap humidity; others
have waxy leaves to retain moisture.

Thus, a
special community of plants that has adapted to harsh conditions develops
on some ultra-mafic soils. The communities are called Serpentine Barrens
or Serpentine Woodlands, because they are often found and studied on serpentine
rock. Often these communities have an unusually high percentage of rare
plants - rare either because they have evolved unusual adaptations to the
ultra-mafic soils, or simply because they are out-competed by other plants
in more favorable environments. Track Rock Gap is special not only because
of the Fringed Gentian, but also because of the presence of Solidagao
bicolor (Silver-rod) and Carex purpurifera (Purple Sedge) and
the abundance of at least two other gentians - Gentiana saponaria
(Soapwort Gentian) and Gentianella quinquefolia (Stiff Gentian).
For a list of typical plants found in such communities in low elevation
Blue Ridge environments, see the NatureServe description below.

Global
Heritage Status Rank: G1Global Heritage Status Rank Reasons: This serpentine woodland is
described from a small number of sites at low elevations in the Southern
Blue Ridge of Georgia and North Carolina. Serpentine (or other ultramafic)
lithology is necessary for development of this community type, and there
are very limited exposures of suitable rocks in the Southern Blue Ridge.
Several of the known locations where this community occurred have been destroyed
by mining or altered by logging. None of the known locations have protected
status.

Global Range Comments: This community occurs in the Southern Blue
Ridge mountains of North Carolina and extreme northern Georgia.